This invention relates to failure monitoring and alarming apparatus which utilizes common control circuitry to carry out a plurality of failure alarm functions for each of a plurality of digital groups, of time division multiplexed channels, that are time multiplexed together on to a common transmission link.
In the past, pulse code modulation (PCM) digital data terminals have typically performed failure monitoring and alarming tasks (as well as framing detection, reframing, etc.) on a per "digroup" basis -- a digroup or digital group comprising a plurality of time division multiplexed PCM messages and multiplexed framing and signaling bits. For example, the failure alarm circuitry for the Bell System's D2 Channel Bank comprises an alarm control circuit and a carrier group alarm, each of which is distributed over a single digital group. The alarm control unit senses failures which occur in the local terminal, in the far end terminal, and in the Tl lines joining the two terminals. The carrier group alarm circuit functions under the direction of the alarm control unit. Any activity by the alarm control which reflects an out-of-frame condition for the digroup, supply voltage failure, circuit failure at either terminal, or line failures causes the carrier group alarm to initiate a sequence of events which results in the stopping of service charges and the withdrawal of the affected digroup from service for the duration of the trouble. The per digroup partitioning of the failure alarm (and other) functions has heretofore resulted in efficient terminal design.
With increasing digital traffic, it is not uncommon now to find proposals for multiplexing a plurality of digroups for transmission to a remote location over a common transmission facility or alternatively for multiplexing a plurality of received digroups on to a common bus at a switching center. These two cases are somewhat analogous and present the same problem with regard to failure alarm. Conventional practice would suggest carrying out the failure alarm functions on a per digroup basis using plural alarming circuits to respectively monitor the plurality of multiplexed digroups and their associated circuits. The obvious disadvantage of this approach is, of course, its complexity and costly redundancy in failure alarm circuitry.